Yusuke Kasai

2-Methoxy-2-(1-naphthyl)propionic acid, a powerful chiral auxiliary for enantioresolution of alcohols and determination of their absolute configurations by the 1H NMR anisotropy method

Abstract Racemic 2-methoxy-2-(1-naphthyl)propionic acid ( 1 , MαNP acid) was enantioresolved as its esters derived from various chiral alcohols. For example, a diastereomeric mixture of esters prepared from (±)- 1 and (1 R ,3 R ,4 S )-(−)-menthol was easily separated by HPLC on silica gel yielding esters (−)- 2a and (−)- 2b , the separation factor α=1.83 being unusually large. The 1 H NMR chemical shift differences, Δδ=δ( R )–δ( S ), between diastereomers 2a and 2b , are much larger than those of conventional chiral auxiliaries, e.g. Mosher’s MTPA and Trost’s MPA acids. This acid 1 is therefore very powerful for determining the absolute configuration of chiral alcohols by the 1 H NMR anisotropy method. Solvolysis of the separated esters yielded enantiopure acids ( S )-(+)- 1 and ( R )-(−)- 1 , which are useful for enantioresolution of racemic alcohols.

Total Synthesis of (−)-Corilagin

The synthesis of corilagin was achieved by the integration of the development of the oxidative coupling of the symmetrically protected gallates and the temporarily ring-opened synthetic route for the 3,6-hexahydroxydiphenoyl (HHDP) bridge. This is the first total synthesis of the 1C4/B-ellagitannins, which contain ring-flipped glucose.

High-Yield Total Synthesis of (−)-Strictinin through Intramolecular Coupling of Gallates

This paper describes a total synthesis of (-)-strictinin, an ellagitannin that is 1-O-galloyl-4,6-O-(S)-hexahydroxydiphenoyl (HHDP)-β-D-glucose. In the study, total efficiency of the synthesis was improved to produce a 78% overall yield in 13 steps from D-glucose. In the synthesis, formation of the 4,6-(S)-HHDP bridge including the 11-membered bislactone ring was a key step, in which intramolecular aryl-aryl coupling was adopted. The coupling was oxidatively induced by CuCl2-n-BuNH2 with perfect control of the axial chirality, and the reaction conditions of this coupling were optimized thoroughly to achieve the quantitative formation of the bridge.

Orientation of Fluorinated Cholesterol in Lipid Bilayers Analyzed by 19F Tensor Calculation and Solid-State NMR

6-F-cholesterol was reported to exhibit biological and interfacial properties similar to unmodified cholesterol. We have also found that 6-F-cholesterol mimicked the cholesterol activity observed in the systems of amphotericin B and lipid rafts. However, to use 6-F-cholesterol as a molecular probe to explore molecular recognition in membranes, it is indispensable to have detailed knowledge of the dynamic and orientation properties of the molecule in membrane environments. In this paper, we present the molecular orientation of 6-F-cholesterol (30 mol %) in dimyristoylphosphatidylcholine (DMPC) bilayers revealed by combined use of 19F chemical shift anisotropy (CSA), 2H NMR, and C-F rotational echo double resonance (REDOR) experiments. The axis of rotation of 6-F-cholesterol was shown to be in a similar direction to that of cholesterol in DMPC bilayers, which is almost parallel to the long axis of the molecular frame. The molecular order parameter of 6-F-cholesterol was determined to be ca. 0.85, which is within the range of reported values of cholesterol. These findings suggest that the dynamic properties of 6-F-cholesterol in DMPC are quite similar to those of unmodified cholesterol; therefore, the introduction of a fluorine atom at C6 has virtually no effect on cholesterol dynamics in membranes. In addition, this study demonstrates the practical utility of theoretical calculations for determining the 19F CSA principal axes, which would be extremely difficult to obtain experimentally. The combined use of quantum calculations and solid-state 19F NMR will make it possible to apply the orientation information of 19F CSA tensors to membrane systems.

Total Synthesis of (+)‐Davidiin

Nature occasionally presents us with unusual chemical structures. Although thermodynamic effects regulate the conformation of d-glucopyranose to a C1 form with more substituents in an equatorial than in an axial position (equatorial-rich form), nature allows the existence of a C4 or a twist-boat conformation with more substituents in an axial than in an equatorial position (axial-rich) of glucose through the introduction of a bridge. This peculiar structure sporadically appears in ellagitannins, a structurally and biologically diverse class of hydrolysable tannins. Ellagitannins with C4 and twist-boat conformers are designated as axial-rich ellagitannins in this communication; (+)-davidiin (1) is a typical example.

Ion channel complex of antibiotics as viewed by NMR

Amphotericin B (AmB) exerts its pharmacological effects by forming a barrel-stave assembly in fungal membranes. To examine the interaction between AmB and ergosterol or cholesterol, 13C- and 19F-labeled covalent conjugates were prepared and subjected to solid-state NMR measurements. Using rotor-synchronous double resonance experiments such as REDOR and RDX, we estimated the distance between the fluorine atom and its nearest carbon in the heptaene moiety to be less than 8.6 Å, indicating that the B ring of ergosterol comes close to the AmB polyene moiety. Conformational search of the AmB-ergosterol conjugate using the NMR-derived constraints suggested that ergosterol molecules surround the AmB assembly in contrast to the conventional image where ergosterol is inserted into AmB molecules. AmB-AmB bimolecular interaction was examined by using 13C- and 19F-labeld AmBs in dimyritoylphosphatidylcholine membrane without sterols. 13C-19F dipolar interactions deriving from both head-to-head and head-to-tail orientations were observed in the REDOR experiments. The interactions between AmB and acyl chains of the phospholipid were also detected.

Total Synthesis of (−)-Thallusin: Utilization of Enzymatic Hydrolysis Resolution

(-)-Thallusin, isolated from a marine bacterium, is the only known natural product to act as an algal morphogenesis inducer. Because (-)-thallusin can only be obtained in exceedingly limited amounts from microbial cultivation, a synthetic supply of this compound is highly desirable. Here, we describe a novel synthetic pathway to (±)-thallusin and the first asymmetric synthesis of (-)-thallusin utilizing the enzymatic hydrolysis resolution with the combination of lipase PS-30 and lipase M Amamo-10.

Glutathione Ethylester, a Novel Protein Refolding Reagent, Enhances both the Efficiency of Refolding and Correct Disulfide Formation

Protein refolding constitutes a crucial process for recombinant proteins. We report here on the development of a multifunctional refolding additive, glutathione ethyl ester (GSHEE), prepared from a redox reagent glutathione and an amino acid ethyl ester, an aggregation suppressor. Compared to glutathione, GSHEE showed 3.2-fold higher efficiency for the refolding yield of hen egg lysozyme. More importantly, a low concentration of GSHEE is more effective for refolding than conventional additives, such as amino acid ethyl esters by two orders of magnitude. The high potency of GSHEE makes it a candidate for use as a refolding additive for use in conjunction with reduced and denatured proteins.

Synthesis of 6-F-ergosterol and its influence on membrane-permeabilization of amphotericin B and amphidinol 3

Two well-known antifungals, amphotericin B (AmB) and amphodinol 3 (AM3), are thought to exert antifungal activity by forming ion-permeable channels or pores together with sterol molecules. However, detailed molecular recognitions for AmB-sterol and AM3-sterol in lipid bilayers have yet to be determined. Toward (19)F NMR-based investigation of the molecular recognition underlying their potent antifungal activity, we synthesized 6-fluoro-ergosterol in five steps via ring opening of (5α,6α)-epoxide of ergosterol acetate with using novel combination of TiF(4) and n-Bu(4)N(+)Ph(3)SiF(2)(-). Then we evaluated its activity of promoting pore formation of AmB and AM3, and found that pore formation of AmB was barely promoted by 6-F-ergosterol in clear contrast to the dramatic promotion effect of unmodified ergosterol, whereas AM3 activity was markedly enhanced in the presence of 6-F-ergosterol, which was comparable to that of unmodified ergosterol. These results indicate that the introduction of an F atom at C6 position of ergosterol plays an inhibitory role in interacting with AmB, but it is not the case with AM3.

Total Synthesis of Polycavernosides A and B, Two Lethal Toxins from Red Alga

Polycavernosides A and B are glycosidic macrolide natural products isolated as the toxins causing fatal human poisoning by the edible red alga Gracilaria edulis (Polycavernosa tsudai). Total synthesis of polycavernosides A and B has been achieved via a convergent approach. The synthesis of the macrolactone core structure is highlighted by the catalytic asymmetric syntheses of the two key fragments using hetero-Diels-Alder reaction and Kiyooka aldol reaction as the key steps, their union through Suzuki-Miyaura coupling, and Keck macrolactonization. Finally, glycosylation with the l-fucosyl-d-xylose unit and construction of the polyene side chain through Stille coupling completed the total synthesis of polycavernosides A and B.